SEOUL (AFP) - North Korea fired another short-range missile on Friday and threatened fresh steps if world powers impose sanctions for its nuclear test, amid signs it may be readying a new long-range launch. With US and South Korean troops on high alert at the border, Chinese fishing boats were reported to be leaving the area in the Yellow Sea that was the scene of deadly naval clashes in 1999 and 2002 between the two Koreas. The communist North, which has warned it could launch an attack on the South, vowed to respond to any fresh sanctions imposed by the United Nations. If the UN Security Council provokes us, our additional self-defence measures will be inevitable, the Norths foreign ministry said in a statement carried by official media. The world will soon witness how our army and people stand up against oppression and despotism by the UNSC and uphold their dignity and independence. Tensions have been running high since Kim Jong-Ils regime tested a nuclear bomb on Monday for the second time and renounced the armistice that ended the Korean War in 1953. In Washington, two US defence officials said that satellite photos suggest that North Korea may be preparing to launch a long-range ballistic missile. Vehicle movements at a missile site in North Korea resemble work done before North Korea fired a long-range rocket last month, the officials told AFP on condition of anonymity. The UN Security Council unanimously condemned last months missile launch. In response, North Korea stormed out of a US-backed six-nation disarmament deal. The Council has been discussing a potential resolution stronger than last months statement to condemn the Norths nuclear test. But it was not yet clear if that would include new sanctions. This is quite a complicated discussion, Britains UN ambassador John Sawers said Thursday. We need some time. The Council was holding expert-level talks on Friday on how to move forward. South Korea and the United States put their troops on the Korean peninsula on higher alert on Thursday, and Seouls defence ministry said forces were keeping a close watch on the land and sea border with the North. North Korea test-fired another missile off its east coast Friday, the sixth this week, according to South Koreas Yonhap news agency. There was no immediate confirmation but the agencys reports of five launches earlier this week were later confirmed by Pyongyang. US Defence Secretary Robert Gates, en route to a regional security meeting in Singapore, accused the North of very provocative, aggressive actions but tried to play down the threat. Gates said he was unaware of any unusual troop movements in the North, which has around 1.1 million soldiers, compared with 680,000 South Korean and 28,500 US troops south of the border. I dont think there is a need for us to reinforce our military presence in the South. Should the North Koreans do something extremely provocative militarily, then we have the forces to deal with it, he added. The North may take further steps following its latest verbal statement, which aims to send a strong warning to the Security Council, said Professor Yang Moo-Jin at Seouls University of North Korean Studies. The North may put its military on a war footing, test-fire a long-range missile and restart the plutonium reprocessing facilities at Yongbyon, he told AFP. The North could also stage a third nuclear test but this would come much later than the other steps, Yang said. In a possible sign of trouble ahead, Chinese fishing boats were leaving the tense border area in the Yellow Sea, with the number of vessels more than halving on Thursday, South Koreas defence ministry said. As this could be a signal foreboding a possible provocation by the North, we are watching the situation closely, ministry spokesman Won Tae-Jae said. Pyongyang warned Wednesday that it could not guarantee the safety of US or South Korean ships after Seoul said it was joining a US-led international effort to stop the trade in weapons of mass destruction. Many experts believe, however, that the North is not yet able to deliver a nuclear weapon by missile.